The present invention relates to a method for reducing the tendency of thermal cracked oils to form solids that coat and plug process equipment. This method consists of reducing the time that the thermally cracked oil spends in the temperature range of 450 to 615.degree. F. (232 to 324.degree. C.), particularly 500 to 580.degree. F. (260 to 304.degree. C.). This is often most conveniently achieved by raising the temperature in the part of the process of highest residence time above 580.degree. F. and preferably higher than 615.degree. F.
It is well known that during the refining of petroleum oils that when the oil has been thermally cracked, the oil has a tendency to form carbonaceous, insoluble solids that coat and plug process equipment. The deposition of solids on process equipment is called fouling and the solids are called foulant. Example thermal cracking processes in a refinery include delayed coking, Fluid Coking, flexicoking, visbreaking, and gas oil thermal cracking. Examples of process equipment downstream of these thermal cracking processes are heat exchangers, reboilers, fractionators, and hydrotreater reactors. Often the carbonaceous solid has a puffed appearance that is called "popcorn coke". Even small amounts of carbonaceous solids deposited on the surface of process equipment can greatly reduce the efficiency of refinery process equipment by reducing heat transfer. Large amounts of carbonaceous solids can result in high-pressure drop which reduces throughput so much that the unit has to be shut down for cleaning. Not only does this result in high cleaning cost but the even higher loss in revenue when the operation of the unit is interrupted and cannot process oil.